Postgres Listen/Notify based Notifier

Caveats

The notifications system is built on PostgreSQL's LISTEN/NOTIFY functionality. Notifications are only delivered after a transaction completes and are de-duplicated before publishing. Most applications run Ecto in sandbox mode while testing. Sandbox mode wraps each test in a separate transaction which is rolled back after the test completes. That means the transaction is never committed, which prevents delivering any notifications. To test using notifications you must run Ecto without sandbox mode enabled.